Compressor Unit and Use of a Cooling Medium

ABSTRACT

A compressor unit comprises a centrifugal compressor for compressing a gas, having a rotor with one or more compressor impellers, and an electric motor having a stator and a rotor, for driving the rotor of the compressor. The compressor and the electric motor are accommodated in a common gas-tight housing which is provided with a gas inlet and a gas outlet. The rotor of the compressor and the rotor of the electric motor are arranged on a common rotor shaft which is mounted in magnetic bearings. The rotor shaft comprises a single unit and is mounted in two radial magnetic bearings, each in the vicinity of one end of the common rotor shaft, and one axial magnetic bearing, which is arranged in the vicinity of one of the radial bearings.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International Application No. PCT/EP2007/051393, filed Feb. 13, 2007 and claims the benefit thereof. The International Application claims the benefits of European application No. 06006069.6 filed Mar. 24, 2006, both of the applications are incorporated by reference herein in their entirety.

FIELD OF INVENTION

The invention relates to a compressor unit for compression of a pumping medium, in particular for underwater operation, comprising a compressor and an electric motor which comprises a stator and a rotor, with the stator of the motor being connected to a separate cooling arrangement and being cooled by means of the separate cooling arrangement, which cooling arrangement has a cooling medium. In addition, the subject matter of the invention covers the use of a special cooling medium for cooling a stator of a motor of a compressor unit for operation under water.

BACKGROUND OF THE INVENTION

Recent developments in the field of compressor design have also been concentrated on undersea arrangements of large compressors which are intended to be used for the pumping of natural gases.

Because of the particular operating conditions, in particular because of the greatly restricted accessibility both for maintenance purposes and by means of supply lines, the specialists are confronted with major requirements. The relevant environmental regulations forbid any exchange of substances between the equipment to be installed and the surrounding sea water. Furthermore, sea water is an aggressive medium and extreme pressure and temperature conditions can be found at the various depths in the sea. A further requirement is that the equipment should on the one hand have an extremely long life and on the other hand must be designed to be virtually free of maintenance. An additional exacerbating factor is not-inconsiderable contamination of the medium to be pumped which in some cases is chemically aggressive.

A compressor unit of the abovementioned type is already known from international patent application WO 02/099286 A1. The compressor unit described there provides, for cooling purposes, that a portion is tapped off from the pumping medium, generally natural gas, in the area of an overflow from the radial stages of the compressor and is used to pass around the components to be cooled, in such a way that the heat losses, which are in the order of magnitude of 100-200 kW, are dissipated with the cold medium to be pumped.

This concept for cooling the compressor unit is particularly advantageous since the pumping medium which must be transported in any case is used to dissipate the heat losses and there is no need for any additional media exchange between the compressor unit and further components of the environment. However, this procedure results in particular difficulties owing to the aggressive chemistry of the media to be pumped.

The pumping medium is frequently heavily contaminated and can adversely affect the operation of sensitive components because it flows around them. For example, the bearings, the axial bearings and the radial bearings are therefore encapsulated in such a way that no substances are exchanged between the surrounding area and these components. This means that magnetic bearings must be used. This also applies to the rotor and to the stator, which are protected against the aggressive pumping media in a similar manner by means of encapsulation. While it is sufficient to cool the rotor by passing flow around it, further cooling measures are required to dissipate the heat losses for the stator. One solution from the prior art provides for the stator to be provided with cooling channels, with the pumping medium flowing through these channels for cooling purposes. This involves the risk of the contamination in the pumping medium adversely affecting the flow through the channels during the course of operation, and possibly also blocking them. A further risk is the generally porous insulation of the stator which absorbs part of the pumping medium in the area where it flows around, while the pumping medium is in contact with the stator as a cooling medium, in such a way that, in the event of a sudden pressure drop, for example in the case of an interruption in operation, this leads to explosive expansion of the absorbed medium in the pores of the insulation, which is in consequence destroyed.

The described disadvantages with their high risks for the availability of a compressor unit are totally unacceptable in particular for underwater operation, for example for the pumping of natural gas.

SUMMARY OF INVENTION

The invention is therefore based on the object of providing a cooling arrangement for the stator of an electric motor of a motor-driven compressor unit, in particular for undersea operation, which on the one hand offers excellent operational reliability and on the other hand does not require any substances to be exchanged with the environment during operation.

A compressor unit as claimed in claim 1 and the use of a pentaerythritol tetracarboxylic acid ester as cooling medium according to claim 11 are proposed in order to solve this problem. The dependent claims each contain advantageous developments of the invention.

One particularly advantageous variant for operation of the cooling arrangement is to use a pentaerythritol tetracarboxylic acid ester as a cooling medium for cooling the stator of the motor during operation under water.

In addition, the high flammability of the pentaerythritol tetracarboxylic acid ester and the reduced risk of fire associated with it are a major advantage of use according to the invention. Furthermore, the low corrosion characteristic and the excellent compatibility with the insulating materials used in these areas, such as mica strips and epoxy resin and/or impregnation resins, are further factors.

The ester liquids and the “Midel” or “Shell Fluid 4600” may be mentioned as examples of pentaerythritol tetracarboxylic acid esters. These liquids are respectively not only an excellent insulator and compatible with the surrounding materials but also comply with the stringent fire protection requirements and stringent requirements for environmental compatibility, since, in particular, they can be classified as not water-contaminating.

In particular, the sensitivity of the stator to contamination is taken into account if the cooling arrangement has a closed circuit in which the cooling medium (pentaerythrityl tetracarbonate) circulates. It is worthwhile in this case for the cooling arrangement to have a heat exchanger which is connected to the cooled stator by means of a feed line and a return line, with the cooling arrangement being designed such that the cooling medium (pentaerythritol tetracarboxylic acid ester) circulates between the heat exchanger, the return line, the stator and the feed line.

The circulation can be driven by means of natural convection, thus resulting in a natural circulation of the cooling medium between the abovementioned components. However, preferably, the cooling medium circuit is operated in a forced-circulation mode by means of a pump. No phase exchange should take place in this case. This ensures a particularly wide thermal range of operation.

For the purposes of the invention, separate cooling for the stator of the electric motor of the compressor unit, on the one hand, and another cooling system for the further elements of the compressor unit, on the other hand, are particularly expedient. The separation of the cooling from the cooling system is appropriate for the particular requirements for the heat dissipation from the stator of a compressor unit of this generic type.

The cooling system which, inter alia, cools the compressor and the rotor of the motor as well particularly advantageously provides the pumping medium as the cooling medium, as a result of which the heat losses are dissipated with the pumping medium to be compressed. This is particularly advantageous for undersea pumping of natural gas, since this is generally relatively cold.

It is particularly worthwhile for the pumping medium to flow around the rotor in the open circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in more detail in the following text using one specific exemplary embodiment and with reference to a drawing, in which:

FIG. 1 shows a schematic illustration of a longitudinal section through a compressor unit according to the invention.

DETAILED DESCRIPTION OF INVENTION

FIG. 1 shows, schematically, a section along a compressor unit 1 according to the invention which has, as major components, a motor 2 and a compressor 3 in a gas-tight housing 4. The housing 4 accommodates the motor 2 and the compressor 3. The housing 4 is provided with an inlet 6 and an outlet 7 in the area of the junction between the motor 2 and the compressor 3, with the fluid to be compressed being sucked in through the inlet 6 by means of a suction connecting stub 8, and with the compressed fluid flowing out through the outlet 7.

The compressor unit 1 is arranged vertically during operation, with a motor rotor 15 of the motor 2 above a compressor rotor 9 of the compressor 3 being combined to form a common shaft 19 which rotates about a common vertical rotation axis 60.

The motor rotor 15 is borne in a first radial bearing 21 at the upper end of the motor rotor 15.

The compressor rotor 9 is borne by means of a second radial bearing 22 in the lower position.

An axial bearing 25 is provided at the upper end of the common shaft 19, that is to say at the upper end of the motor rotor 15. The radial bearings and the axial bearing operate electromagnetically and are each encapsulated. In this case, the radial bearings extend around the respective bearing point of the shaft 19 in the circumferential direction and in this case are circumferential through 360° and are undivided.

The compressor 3 is in the form of a centrifugal compressor and has three compressor stages 11 which are each connected by means of an overflow 33. The pressure differences which result across the compressor stages 11 ensure that there is a thrust on the compressor rotor 9 which is transmitted on the motor rotor 15 and is directed against the force of gravity from the entire resultant rotor comprising the compressor rotor 9 and the motor rotor 15, thus resulting in a very high degree of thrust matching during rated operation. This allows the axial bearing 25 to be designed to be comparatively smaller than if the rotation axis 60 were to be arranged horizontally.

The electromagnetic bearings 21, 22, 25 are cooled to the operating temperature by means of a cooling system 31, with the cooling system 31 providing a tap 32 in an overflow of the compressor 3. A portion of the pumping medium, which is preferably natural gas, is passed from the tap 32 by means of pipelines through a filter 35, and is then passed through two separate pipelines to the respective outer bearing points (first radial bearing 21 and fourth radial bearing 24 as well as the axial bearing 25). This cooling by means of the cold pumping medium 80 saves additional supply lines.

The motor rotor 15 is surrounded by a stator 16 which has encapsulation 39 such that the aggressive pumping medium 80 does not damage the windings of the stator 16. The encapsulation is in this case preferably designed such that it can contribute to the full operating pressure. This is also because a separate cooling arrangement 40 is provided for the stator, in which cooling arrangement 40 a dedicated cooling medium (pentaerythritol tetracarboxylic acid ester) 56 circulates. A pump 42 in this case ensures circulation via a heat exchanger 43.

At least the encapsulation 39 is designed such that the section which extends between the stator 16 and the motor rotor 15, while having a thin wall thickness, is nevertheless able to withstand the design pressure when the stator cooling arrangement 40 is completely filled by means of the cooling medium 56. This makes it possible to avoid relatively high eddy current losses in this area, thus improving the efficiency of the overall arrangement.

The compressor rotor 9 expediently has a compressor shaft 10 on which the individual compressor stages 11 are mounted. This can preferably be done by means of a thermal shrink fit. An interlock, for example by means of polygons, is likewise possible. Another embodiment provides for different compressor stages 11 to be welded to one another, thus resulting in an integral compressor rotor 9.

FIG. 2 shows the motor rotor 15, the stator 16 and the cooling arrangement 40. The cooling arrangement 40 has a cooling circuit 50 which extends through cooling channels 51, collecting areas 52 arranged on both sides of the cooling channels 51, into lines which connect these collecting areas, specifically a feed line 53 and a return line 54, as well as a condenser 55 arranged between the feed line 53 and the return line 54. The cooling medium 56, namely a pentaerythritol tetracarboxylic acid ester with the trade name “Midel”, starts to flow in cooling channels 51 of the stator 16, flows through the feed line 53 into the heat exchanger 55 where the cooling medium 56 is cooled flows through the return line 54 into a collecting area 52 which is located at the return end of the cooling channels 51. The circuit is closed. The temperature difference between the feed and the return is preferably 10 K. The heat exchanger is located geodetically at the highest point (height difference ΔH), thus assisting natural convection. A pump 42 is arranged in the return. The stator is encapsulated, and cooling by means of the pumping medium 80 which flows around the rotor 15 takes place in a gap between the rotor 15 and the stator 16. 

1.-15. (canceled)
 16. A compressor unit for underwater compression of a pumping medium, comprising: a compressor and an electric motor having a stator and a rotor; and a separate cooling arrangement connected to and cooling the motor stator wherein the cooling arrangement has a cooling medium comprising pentaerythritol tetracarboxylic acid ester.
 17. The compressor unit as claimed in claim 16, wherein the carboxylic acid residue is linear and singly methyl-branched.
 18. The compressor unit as claimed in claim 17, wherein that the carboxylic acid residue has an average C-number≧7.
 19. The compressor unit as claimed in claim 18, wherein the cooling medium circulates in a closed circuit within the cooling arrangement.
 20. The compressor unit as claimed in claim 16, wherein the cooling arrangement has a heat exchanger connected to the cooled stator via a feed line and a return line, with the cooling arrangement designed such that the cooling medium circulates between the heat exchanger, the return line, the stator and the feed line.
 21. The compressor unit as claimed in claim 20, wherein a pump is provided in the return line and pumps the cooling medium to produce a forced circulation.
 22. The compressor unit as claimed in claim 21, wherein the cooling arrangement is designed for the maximum operating pressure.
 23. The compressor unit as claimed in claim 15, wherein the compressor, the bearings of the compressor unit and the rotor of the motor are connected to and cooled by a further cooling system.
 24. The compressor unit as claimed in claim 16, wherein the cooling system has a cooling medium which is the pumping medium.
 25. The compressor unit as claimed in claim 24, wherein the pumping medium is natural gas.
 26. The compressor unit as claimed in claim 25, wherein the cooling medium flows around the rotor.
 27. A method of cooling a motor stator of an underwater compressor unit, comprising: providing a separate cooling arrangement; connecting the cooling arrangement to and cooling the motor stator with the cooling arrangement wherein the cooling arrangement has a cooling medium comprising pentaerythritol tetracarboxylic acid ester.
 28. The method as claimed in claim 27, wherein a carboxylic acid residue of the pentaerythritol tetracarboxylic acid ester is linear and singly methyl-branched.
 29. The method as claimed in claim 28, wherein the carboxylic acid residue has an average C number≧7.
 30. The method as claimed in claim 29, wherein the cooling arrangement is operated in a forced-circulation mode. 